Endocrine Abstracts

Background: The level of 25OH vitamin D (25OHD) which separates deficiency from sufficiency is heavily deabted. With decreasing 25OHD levels, typical biochemical derangements set in, such as increasing parathyroid hormone (PTH) and alkaline phosphatase (ALP) levels, and decreasing phosphate and calcium levels. In adults, serum concentrations of PTH reportedly start to rise when serum 25OHD levels drop below 30 μg/l. Such data are scarce in children.

Objective and hypotheses: To determine the concentration of serum 25OHD at which PTH or other bone metabolites start to derange.

Methods: Retrospective audit of blood results from paediatric patients attending our metabolic bone clinic and from our hospital’s clinical chemistry dataset. Only patients who had 25OHD, PTH, calcium, phosphate and ALP measured simultaneously were included. Patients with diseases,medications that affect the physiological relationship between the examined biochemical variables, or those with insufficient clinical information were excluded.

Results: The final dataset included 206 children (median age 9.69y). 25OHD ranged from 0.2 to 66.5 μg/l. Of those with 25OHD levels <15 μg/l, 61.9, 33.6, 38.5, 17.8 and 75.2% had biochemical hyperparathyroidism (PTH>50 pg/ml), high ALP (>1000 U/l), hypophosphataemia (<1.3 mmol/l), hypocalcaemia (<2.2 mmol/l) and any biochemical derangement, respectively. These numbers increased to 71.2, 37.2, 40.4, 19.1 and 82.9% when a 25OHD level of 12.5 μg/l was chosen. All 15 patients with confirmed vitamin D deficiency rickets had 25OHD levels <12.5 μg/l.

Conclusion: PTH levels in this heterogeneous group of patients increased sharply at 25OHD levels <15 μg/l. Nevertheless, a large number of children with vitamin D levels <15 μg/l have no bone metabolic derangement which could be related to a shorter duration of vitamin D deficiency and variable calcium intake.